Wafer inspection systems generally use a vacuum chuck to hold a silicon wafer during inspection. The advantage of this method is that the wafer surface can be as flat as the machined surface of the vacuum chuck. Systems in which a wafer rests on a vacuum chuck or other flat surface are not particularly flexible and require complex and expensive staging mechanisms to move the surface on which the wafer rests and/or the imaging system used to capture images of the wafer. Other wafer inspection systems float silicon wafers on a cushion of air. This type of system suffers from positioning errors in that there is inherent variation in the position a wafer that is not positively gripped. What is more, the contact between the edge of a wafer and a rotary positioning mechanism often gives rise to small chips and other defects at the edge that may contaminate the entire wafer, especially as the air cushion provides a means for distributing contaminants over the surface of the wafer. Yet other systems use robotic devices or manipulators that rotate the wafer about multiple, orthogonal axes to gain access to the backside of a wafer. But because wafers do flex somewhat as a result of such manipulation, it is possible to damage the wafer itself and/or the microelectronic structures formed thereon.
One solution to the problems faced by the inspection systems described above is to grip a silicon wafer by its edges and to rotate the wafer about a vertical axis such that an imaging mechanism may capture images of the entire backside of the wafer. This method limits the stresses applied to a wafer and avoids damage thereto. However, in some instances and because gripping a wafer only at its edges does not support the entire wafer surface, it is possible for a wafer to sag or deform under the influence of gravity. Where relatively low magnification optics are used to capture images of the wafer, the depth of focus of these optics will generally be large enough to ensure that the surface of the wafer will remain in focus during inspection. But where relatively higher magnification optics are used, it is possible that a significant portion of the wafer surface may at some point fall outside the depth of focus of the inspection optics, thereby resulting in poor performance of the system.
Accordingly, there is need for a method and apparatus for characterizing the sag or deformation of an edge supported wafer and using this characterization to ensure that the optics of an imaging system remain focused on the surface of a wafer during the imaging of the wafer surface.